5.1.4.1Sub-category description -
Natural gas is a fossil fuel extracted, refined and used for various purposes, especially combustion to produce electricity and heat. Like many other natural materials, natural gas contains small amounts of natural mercury impurities, which are mobilized to the biosphere during extraction, refining and combustion. In some regions of the world, natural gas is known to have notable mercury concentrations (depending on geology). Mercury releases may occur during extraction, refining, gas cleaning steps and use (COWI, 2002 and US EPA, 1997b). In some countries, mercury in gas cleaning residues ("condensate" or specific Hg filter rejects) is recovered and marketed as a by-product. In other countries, these residues are collected and treated as hazardous waste. For off-shore gas extraction, initial gas cleaning steps sometimes take place off-shore and may involve the use of cleaning water, which may be discharged on site. The fate of the mercury content observed in natural gas is still poorly understood. This may be considered a major data gap in the description of mercury releases. In most countries, the gas delivered to consumers has been cleaned and contains - at that stage - only little mercury.
-
The natural gas power production process begins with the extraction of natural gas, continues with its treatment and transport to the power plants, and ends with its combustion in boilers and turbines to generate electricity. Initially, wells are drilled into the ground to remove the natural gas. After the natural gas is extracted, it is treated at gas plants to remove impurities such as hydrogen sulphide, helium, carbon dioxide, hydrocarbons moisture, and to some extend mercury (either in general treatment or as mercury-specific filters). Gas cleaning operations may take place off-shore. Pipelines then transport the natural gas from the gas plants to power plants, or via gas supply grids to residential burners, for combustion.
-
Other uses of natural gas include among others synthesis of chemicals, polymer production and carbon black production (black pigment).
-
Mercury is a particular problem for plants producing liquid natural gas (LNG) and in nitrogen rejection units (NRU) as it can cause deformation of aluminium heat exchangers due to mercury amalgamating with the aluminium. Mercury is also a poison for the precious metal catalysts used in many of the reactions used in hydrocarbon processing and many operators set tight limits on the level of mercury in feed materials to crackers. For these reasons, mercury is in some cases removed from the gas with mercury-specific filters (usually fixed bed filters with impregnated pellets). Some filters use absorbents which are deposited as waste (NCM, 2010), while others may be regenerated on-site along with regeneration of moisture filters (UOP, undated). In the latter case, the captured mercury-containing hydrocarbons may be fed from the regeneration filters back into marketed gas or liquid fuel streams, or it may be concentrated in a smaller filter from which the mercury-containing filter material is deposited as waste UOP (undated).
5.1.4.2Main factors determining mercury releases and outputs
Table 5 40 Main releases and receiving media during the life-cycle of extraction, refining and use of natural gas
Phase of life cycle (/use)
|
Air
|
Water
|
Land
|
Products
|
General waste
|
Sector specific treatment/
disposal
|
Extraction /Refining
|
X
|
X
|
X
|
x
|
x
|
X
|
Combustion
|
x
|
|
|
|
|
|
Other uses
|
|
|
|
|
|
|
Notes: X - Release pathway expected to be predominant for the sub-category;
x - Additional release pathways to be considered, depending on specific source and national situation.
-
The most important factors determining releases are the mercury levels in the natural gas and amount of gas extracted, refined or combusted.
-
Most of the mercury in the raw natural gas may be removed during the extraction and/or refining process, including during the removal of hydrogen sulphide (Pirrone et al., 2001). Therefore, natural gas is generally considered a clean burning fuel that usually has very low mercury concentrations.
-
Also, little to no ash is produced during the combustion process at these facilities (US EPA, 1997b). During combustion, since the entire fuel supply is exposed to high flame temperatures, essentially all of the mercury remaining in the natural gas will be volatilized and exit the furnace with the combustion gases through the emissions stack. Gas-fired plants usually have no emissions control devices that would reduce mercury emissions (US EPA, 1997a).
Table 5 41 Overview of activity rate data and mercury input factor types needed to estimate releases from extraction, refining and use of natural gas
Life-cycle phase
|
Activity rate data needed
|
Mercury input factors
|
Extraction /Refining
|
Amount of natural gas produced
|
Concentration of mercury in
extracted gas
|
Combustion/use
|
Amount of natural gas combusted
|
Concentration of mercury
in natural gas combusted
|
-
Detailed estimates of national consumption of different fuel types, in totals and by sector, are available on the International Energy Agency's statistics website http://www.iea.org/stats/.
-
Natural gas combustion: Mercury concentrations in natural gas may vary depending on the local geology, however, mercury concentrations in consumer supplies ("pipeline gas") appear to be generally very low (COWI, 2002 and US EPA, 1997b).
-
Examples of mercury content of wellhead gas are shown in Table 5 -42. The mercury content varies considerable between different regions of the world. It should be noted that it is unclear to what extent the presented data sets represent regions with particularly high mercury content.
Table 5 42 Examples of mercury concentrations in wellhead gas
Notes
|
Range
(g/Nm3)
|
Mean
(g/Nm3)
|
Number
of samples
|
Reference
|
USA wellhead gas (estimated)
|
|
<1 *1
|
|
Wilhelm, 2001
|
Russian Federation,
wellhead gas from oil wells
|
0.05-70 *1
|
2.4 *1
|
48
|
Lassen et al., 2004
|
Russian Federation,
free gas from gas wells
(after primary condensate separator)
|
0.07-14 *1
|
3,4 *1
|
169
|
Lassen et al., 2004
|
San Joaquin Valley, California
|
1.9-21
|
|
|
Bailey et al.,1961 *2
|
Middle East
|
<50
|
|
|
Hennico et al., 1991 *2
|
Netherlands
|
0.001-180
|
|
|
Bingham, 1990 *2
|
South Africa
|
100
|
|
|
Hennico et al.,1991 *2
|
Netherlands
|
0-300
|
|
|
Gijselman, 1991 *2
|
Far East
|
50-300
|
|
|
Hennico et al., 1991 *2
|
Sumatra
|
180-300
|
|
|
Muchlis, 1981;
Situmorang and
Muchlis , 1986 *2
|
Unknown, examples from gas filter industry
|
<0.01-120
|
|
|
UOP, undated
|
Notes *1 The references use the unit g/m3 without indicating whether the volume is normalized to Nm3;
*2 As cited in OilTracers (1999-2004).
Table 5 43 Examples of mercury concentrations in pipeline gas (cleaned and as received at consumers)
Country
|
Mercury concentration
g/m3
|
Reference and notes
|
USA
|
<0.02 - <0.2
|
Wilhelm, 2001; all results below the detection limit of the methods used for different analyses
|
Russian Federation
|
0.03 - 0.1
|
Lassen et al., 2004
|
Denmark
|
<0.1 - 0.8
|
Skårup et al., 2003
|
-
Pirrone et al. (2001) reported that “a reduction of mercury to below 10 g/m3 has to be obtained before the gas can be used”, which may indicate that mercury concentrations in consumer gas quality may be generally below this level in Europe (the geographical area of interest in the study), but that the raw natural gas may sometimes have higher mercury concentrations.
5.1.4.4Examples of mercury in releases and wastes/residues
The significant part of the mercury content of the raw natural gas may generally be separated from the gas into the different gas cleaning waste waters or condensates. Table 5 -44 shows an example of the distribution of mercury in a gas plant in the East Asia without mercury removal filter. The actual distribution will be highly dependent on whether mercury removal beds are installed. The cited paper present data on mercury concentration in 5 different plants, but present mass balance for one plant only. In three plants without mercury removal bed the sales gas contained up to 3000 ng/m3; whereas for the two plants with mercury removal beds the concentration were 10 and 2 ng/m3, respectively. In the specific plant 68% of the mercury ended up in the sales gas. It should be noted that the percentage ending up in the sales gas will be highly dependent on the initial mercury concentration of the raw gas as the target for the cleaning process is a certain concentration in the sales gas.
Table 5 44 Example of distribution of mercury in a gas plant without mercury removal bed (Carnell and Openshaw, 2004)
Process Stream
|
Mercury (Kg/year)
|
Percentage of mercury in raw gas
|
Raw Gas
|
220
|
100 %
|
Acid Gas Removal Vent
|
22
|
10 %
|
Dryer Vent
|
3
|
1 %
|
Condensate
|
45
|
20 %
|
Sales Gas
|
150
|
68 %
|
-
The term gas condensate refers to liquids that can originate at several locations in a gas processing scheme (Wilhelm, 2001). A generic unprocessed condensate is the hydrocarbon liquid that separates in the primary separator, either at the wellhead or at the gas plant. Processed condensate is the C5+ fraction (heavier hydrocarbons) that is a product from a gas separation plant.
Table 5 45 Examples of mercury concentrations in gas condensates
Reference
|
Number
of samples
|
Range
(ppb)
|
Mean
(ppb)
|
SD
|
Notes
|
Olsen et al., 1997 *1
|
4
|
NR
|
15
|
|
Origins not reported
|
Shafawi et al., 1999 *1
|
5
|
9-63
|
30
|
18.6
|
S.E. Asia
|
Tao et al., 1998 *1
|
7
|
15-173
|
40
|
|
Asian
|
Lassen et al., 2004
|
5
|
60-470
|
270
|
270
|
Russian Federation
|
Bloom, 2000 *1
|
18
|
NR
|
3,964
|
11,655
|
Mostly Asian
|
Notes: *1 As cited by Wilhelm (2001); "NR" means not reported.
-
In an example from the Gulf of Thailand the produced water before cleaning in three fields was reported to contain 191-235 ppb, 155 ppb and 11 ppb, respectively (Gallup and Strong, 2006). After treatment with a 0.45 µm filtrate the concentration was reduced to <1-10 ppb. The main part of the produced water from the fields was injected in the fields while a minor part was discharged to the water. As example of the significance of the mercury discharges with production water, 40 – 330 kg Hg/year with an average value of 187 kg Hg/year was released with production water into the Gulf of Thailand between 1991 and 1996 (Chongprasith et al., 2009). In recent years various treatment technologies have been employed to remove the mercury prior to discharge.
-
For pipeline gas, i.e. the gas received by consumers, all mercury inputs may be considered as released to air during use or combustion.
-
Based on the information compiled above on inputs and outputs and major factors determining releases, the following preliminary default input and distribution factors are suggested for use in cases where source specific data are not available. It is emphasized that these default factors are based on a limited data base, and as such, they should be considered preliminary and subject to revisions. In many cases calculating releases intervals will give a more appropriate estimate of the actual releases.
-
The primary purpose of using these default factors is to get a first impression of whether the sub-category is a significant mercury release source in the country. Usually, release estimates would have to be refined further (after calculation with default factors) before any far reaching action is taken based on the release estimates.
a) Default mercury input factors -
Actual data on mercury levels in the particular natural gas extracted, refined and used, will lead to the best estimates of releases.
-
If no indications are available on the mercury concentration in the gas used, a first estimate can be made by using the default input factors selected in Table 5 -46 below (based on the data sets presented in this section). Because concentrations vary so much, it is recommended to calculate and report intervals for the mercury inputs to this source category. The low end default factors have been set to indicate a low end estimate for the mercury input to the source category (but not the absolute minimum), and the high end factor will result in a high end estimate (but not the absolute maximum). If it is chosen not to calculate as intervals, the use of the maximum value will give the safest indication of the possible importance of the source category for further investigation. Using a high end estimate does not automatically imply that actual releases are this high, only that it should perhaps be investigated further.
Table 5 46 Preliminary default input factors for mercury in various natural gas qualities
Gas quality
|
Default input factors;
g Hg/Nm3 gas;
(low end - high end)
|
Raw or pre-cleaned gas
|
2 – 200
|
Pipeline gas (consumer quality)
|
0.03 – 0.4
|
-
Natural gas production data may be given as TJ (Terajoule), which can be converted to the unit needed in the Toolkit, Nm3 (normal cubic meters), by multiplying the TJ number with 25,600 Nm3/TJ (an average gross calorific value of natural gas derived from http://www.iea.org/stats/docs/statistics_manual.pdf, p182).
b) Default mercury output distribution factors
For extraction processes and combustion/use of natural gas, the default factors shown in Table 5 -47 below can be used to calculate an indicative mercury release estimate.
Table 5 47 Preliminary default output distribution factors for mercury from extraction, refining and use of natural gas
Phase of life cycle
|
Output distribution factors, share of Hg input
|
Air
|
Water
|
Land
|
Products *1
|
General waste
|
Sector specific treatment/
disposal
|
Extraction and processing - without mercury removal
|
0.2
|
0.2
|
|
0.5
|
0.1
|
|
Extraction and processing - with mercury removal
|
0.1
|
0.2
|
|
0.1
|
0.6
|
|
Combustion/use
|
1
|
|
|
|
|
|
*1 includes both sales gas and condensate
c) Links to other mercury sources estimation -
No links suggested.
5.1.4.6Source specific main data -
The most important source specific data would in this case be:
-
Measured data or literature data on mercury concentrations in the natural gas extracted, refined and combusted at the source;
-
Amount of natural gas extracted, refined and burned; and
-
Measured data on emission reduction equipment applied on the source (or similar sources with very similar equipment and operating conditions).
-
See also advice on data gathering in section 4.4.5.
5.1.5Other fossil fuels - extraction and use 5.1.5.1Sub-category description -
This category includes extraction and use of other fossil fuels such as peat (which is a very young form of coal) and oil shale. Oil shale is a type of shale from which a dark crude oil can be recovered by distillation. Like other fossil and non-fossil fuels these may contain traces of mercury, which can be mobilized by extraction and combustion.
-
Only limited data have been collected on these potential mercury release sources for this Toolkit version. If no other data can be found during inventory development work, an option is to measure mercury concentrations in the fuel types used and in any residues and releases produced.
5.1.5.2Main factors determining mercury releases and outputs
Table 5 48 Expected release pathways and receiving media during the life-cycle of extraction and use of other fossil fuels
Phase of life cycle
|
Air
|
Water
|
Land
|
Product
|
General waste
|
Sector specific treatment/
disposal
|
Extraction
|
|
|
|
|
|
|
Combustion
|
X
|
x
|
x
|
|
x
|
x
|
Notes: X - Release pathway expected to be predominant for the sub-category;
x - Additional release pathways to be considered, depending on specific source and national situation.
5.1.5.3Example of mercury inputs
Table 5 49 Overview of activity rate data and mercury input factor types needed to estimate releases from extraction and use of other fossil fuels
Life-cycle phase
|
Activity rate data needed
|
Mercury input factor
|
Combustion
|
Amount of fuels used
|
Mercury concentration in fuels used
|
-
Mercury is known to be present in peat and oil shale. For example, one study in North Carolina, USA, reported total mercury concentrations from 40 - 193 ng/g (dry weight) in peat, based on measurement data (Evans et al., 1984).
-
Detailed estimates of national consumption of different fuel types, in totals and by sector, are available on the International Energy Agency's website http://data.iea.org/ieastore/statslisting.asp.
5.1.5.4Examples of mercury in releases and wastes/residues -
No data collected.
5.1.5.5Input factors and output distribution factors a) Default mercury input factors -
Peat: If no other data are available, the mercury concentration mentioned in section 5.1.5.3 above may be used.
-
Oil shale: No factor was developed.
b) Default mercury output distribution factors -
Peat: If nothing else is known, 100% of the mercury in the peat can be considered as released to air (as a rough estimate - minor amounts of mercury may likely follow combustion residues and ashes).
-
Oil shale: No factors were developed for this source sub-category.
c) Links to other mercury sources estimation -
No links suggested.
5.1.6Biomass fired power and heat production 5.1.6.1Sub-category description -
Many countries and regions rely heavily on the combustion of biomass for power and heat production. These sources combust wood, including twigs, bark, sawdust and wood shavings; peat; and/or agricultural residues (such as straw, citrus pellets, coconut shells, poultry litter and camel excretes) (UNEP, 2003). Wood wastes are used for fuel in industry. In the residential sector, wood is used in wood stoves and fireplaces (Pirrone et al., 2001). For this Toolkit, sources within this sub-category include wood-fired boilers, other types of biomass-fired boilers, wood stoves, fireplaces and other biomass burning. For the boilers, it is assumed that reasonably well-operated and maintained power steam generators are employed in order to maximize power output. This section does not address firing of contaminated wood.
-
Biomass is burned in a wide array of devices for power generation ranging from small stoker fired furnaces to large elaborate highly sophisticated boiler/burner systems with extensive air pollution control (APC) devices. The combustion of biomass for power generation takes place predominantly in two general types of boilers (stokers and fluidized bed boilers), which are distinguished by the way the fuel is fed to the system (UNEP, 2003).
-
The stokers fired boilers use a stationary, vibrating or travelling grate on which the biomass is transported through the furnace while combusted. Primary combustion air is injected through the biomass fuel from the bottom of the grate. All these firing systems burn biomass in a highly efficient manner leaving the majority of the ash as a dry residue at the bottom of the boiler (UNEP, 2003).
-
The fluidized bed boilers use a bed of inert material (e.g., sand and/or ash), which is fluidized by injecting primary combustion air. The biomass is shredded and added to the fluidized bed, where it is combusted. The fluidized ash, which is carried out with the flue gas, is commonly collected in a (multi-) cyclone followed by an ESP or baghouse and re-injected into the boiler. None or very little bottom ash leaves the boiler, since all the larger ash particles either remain within the fluidized bed or are collected by the cyclone separator. Thus, almost all the ash is collected as fly ash in the ESP or baghouse (UNEP, 2003).
-
Heating and cooking in residential households with biomass is common practice in many countries. In most cases the fuel of preference is wood, however, other biomass fuels may be used.
-
Biomass for residential heating and cooking is burned in a wide array of devices ranging from small, open pit stoves and fireplaces to large elaborate highly sophisticated wood burning stoves and ovens. The combustion of biomass for household heating and cooking occurs predominantly in devices of increasing combustion efficiency, as the gross national product and the degree of development of countries increase (UNEP, 2003).
5.1.6.2Main factors determining mercury releases and outputs
Table 5 50 Main releases and receiving media from biomass fired power and heat production
Phase of life cycle
|
Air
|
Water
|
Land
|
Product
|
General waste
|
Sector specific treatment/
disposal
|
Combustion
|
X
|
x
|
x
|
|
x
|
x
|
Notes: X - Release pathway expected to be predominant for the sub-category;
x - Additional release pathways to be considered, depending on specific source and national situation.
-
The most important factors determining releases are the mercury levels in the fuel and amount of fuel burned. Mercury in biofuels originates from both naturally present mercury and mercury deposited from anthropogenic emissions (COWI, 2002). For example, trees (especially needles and leaves) absorb mercury from the atmosphere overtime. This mercury is readily released mostly to air when the wood and other biomass are burned (Friedli, H.R. et al., 2001).
-
Mercury releases from wood combustion and other biofuels may be significant in some countries (COWI, 2002). Most of the mercury in the biomass is expected released to the air from the combustion process. A smaller amount of mercury may be released to the ashes or residues, the extent of which depends on the specific material burned, type of combustion device, and any emission controls present.
5.1.6.3Discussion of mercury inputs
Table 5 51 Overview of activity rate data and mercury input factor types needed to estimate releases from biomass fired power and heat production
Activity rate data needed
|
Mercury input factor
|
Type and amount of biomass burned
|
Concentration of mercury in the biomass burned
|
-
The main input factor needed is the concentration of mercury in the wood or other biomass burned at the source and the amount of each type of biomass that is burned.
-
For uncontrolled wood combustion sources, the US EPA developed an emission factor of 0.0021 grams of mercury per metric tons of wood, as burned (i.e., wet weight). Using the assumption that all of the mercury in wood from these uncontrolled sources is emitted to the air, it is estimated that the average concentration of mercury in wood burned in the USA is about 0.002 ppm (US EPA, 1997a and NJ MTF, 2002).
-
An average atmospheric emission factor of 0.0026 g mercury per metric tons burned wood is recommended by the US EPA as the so-called "best typical emission factor" for wood waste combustion in boilers in the USA (US EPA, 1997b).
-
In investigations in the USA, the mercury content of litter and green vegetation from seven locations in the USA ranged from 0.01 – 0.07 mg Hg/kg dry weight (Friedly et al., 2001).
-
According to Danish investigations the mercury content of wood and straw burned in Denmark is in the range of 0.007 - 0.03 mg/kg dry weight (Skårup et al., 2003).
-
Swedish investigations found mercury concentrations of 0.01 - 0.02 mg/kg dry weight in fuel wood; however, concentrations of 0.03 - 0.07 mg/kg dry weight in willow wood were found (Kindbom and Munthe, 1998). In bark, a mercury concentration of 0.04 mg/kg dry weight was found, whereas in fir needles the concentration was 0.3 - 0.5 mg/kg dry weight (Kindbom and Munthe, 1998).
-
Detailed estimates of national consumption of different fuel types, in totals and by sector, are available on the International Energy Agency's website http://data.iea.org/ieastore/statslisting.asp.
5.1.6.4Examples of mercury in releases and wastes/residues -
Although some wood stoves use emission control measures such as catalysts and secondary combustion chambers to reduce emissions of volatile organic compounds and carbon monoxide, these techniques are not expected to affect mercury emissions. However, some wood-fired boilers employ PM control equipment that may provide some reduction. Currently, the four most common control devices used in the USA to reduce PM emissions from wood-fired boilers are mechanical collectors, fabric filters, wet scrubbers, and electrostatic precipitators (ESP’s). Of these controls, the last three have the potential for significant capture of mercury (US EPA, 1997a, US EPA, 2002a and US EPA, 1996).
-
The most widely used wet scrubbers for wood-fired boilers in the USA are venturi scrubbers. No data were identified on the control efficiency of these devices for mercury emissions on wood boilers. However, some control is expected. Fabric filters and ESP’s are also employed on some of these wood boilers. Data were not identified for the control efficiencies of these devices on wood fired boilers. However, based on data from coal combustion plants, collection efficiencies for mercury by FFs may be 50% or more, and efficiencies for ESP’s are likely to be somewhat lower, probably 50% or less (US EPA, 1997a and US EPA, 2002a).
-
The data on mercury releases from wood combustion are limited. A report by the National Council of the Paper Industry for Air and Stream Improvement (NCASI) in the USA provided a range and average emission factor for boilers without ESP’s and for boilers with ESP’s (NCASI, 1995, as cited in US EPA, 1997a). The boilers without ESP’s had a variety of other control devices including cyclones, multiclones, and various wet scrubbers. The average emission factor reported for boilers without ESP’s was 3.5 x 10-6 kg/metric tons of dry wood burned. The average emission factor reported for boilers with ESP’s was 1.3 x 10-6 kg/metric tons of dry wood burned. For combustion of wood scraps in uncontrolled boilers, the US EPA established an average emission factor for mercury emissions (based on four emission tests) of 2.6 x 10-6 kg/metric tons of wet, as-fired wood burned (U.S EPA 1997a).
5.1.6.5Input factors and output distribution factors -
Based on the so far compiled examples of mercury concentrations in biomass and general information on emission reduction system efficiency, the following preliminary default input and distribution factors are suggested for use in cases where source specific data are not available. It is emphasized that the default factors suggested in this Toolkit are based on a limited data base, and as such, they should be considered subject to revisions as the data base grows. The primary purpose of using these default factors is to get a first impression of whether the sub-category is a significant mercury release source in the country. Usually release estimates would have to be refined further (after calculation with default factors) before any far reaching action is taken based on the release estimates.
-
Bearing in mind the large variation presented above on both mercury concentrations in biomass and the efficiency of emission reduction systems on mercury, the use of source specific data is the preferred approach, if feasible.
a) Default mercury input factors
Fossil fuels, if used, will also contribute to mercury inputs, but fossil fuels are accounted for in other sub-categories.
Table 5 52 Preliminary default input factors for mercury in coal for energy production
Material
|
Default input factors;
g mercury per metric ton of biomass (dry weight);
(low end - high end)
|
Biomass used in combustion (principally wood)
|
0.007 - 0.07
|
b) Default mercury output distribution factors
Table 5 53 Preliminary default distribution factors for mercury outputs from pulp and paper production (with own pulp production)
Emission reduction device
|
Distribution factors, share of Hg input
|
|
Air
|
Water
|
Land
|
Products
|
General waste
|
Sector specific treatment/
disposal
|
None
|
1
|
|
|
|
|
| -
c) Links to other mercury sources estimation -
No links suggested.
5.1.6.6Source specific main data -
The most important source specific data would in this case be:
-
Measured data or literature data on the mercury concentrations in the types of biomass combusted at the source;
-
Amount of each type of biomass burned; and
-
Measured data on emission reduction equipment applied on the source (or similar sources with very similar equipment and operating conditions).
-
See also advice on data gathering in section 4.4.5.
5.1.6.7Summary of general approach to estimate releases -
The overall approach to estimate releases of mercury to each pathway from biomass combustion is as follows:
Input factor
(concentration of Hg
in the biomass burned at plant)
|
*
|
Activity rate
(amount of each type of biomass burned per year)
|
*
|
Distribution factor
for each pathway
|
and the total releases are the sum of the releases to each pathway.
5.1.7Geothermal power production 5.1.7.1Sub-category description -
Geothermal power plants exploit elevated underground temperatures for energy production and are mostly situated in areas with special geothermal activity, sometimes in areas with volcanic activity. These power plants are either dry-steam or water-dominated. For dry-steam plants, steam is pumped from geothermal reservoirs to turbines at a temperature of about 180 ºC and a pressure of 7.9 bars absolute. For water-dominated plants, water exists in the producing strata at a temperature of approximately 270 ºC and at a pressure slightly higher than hydrostatic. As the water flows towards the surface, pressure decreases and steam is formed, which is used to operate the turbines (US EPA, 1997a).
-
The mercury releases from geothermal power plants are caused by the mobilisation of mercury naturally occurring under these geological conditions.
5.1.7.2Main factors determining mercury releases and mercury outputs
Table 5 54 Main releases and receiving media during geothermal power production
Phase of life cycle
|
Air
|
Water
|
Land
|
General waste
|
Sector specific treatment/
disposal
|
Geothermal power production
|
X
|
|
|
|
|
Notes: X - Release pathway expected to be predominant for the sub-category;
x - Additional release pathways to be considered, depending on specific source and national situation.
-
Mercury is released to the air from geothermal power plants, and possibly to other media. Mercury emissions at geothermal power plants are released via two outlet types: off-gas ejector and cooling towers (US EPA, 1997a).
5.1.7.3Discussion of mercury inputs
Table 5 55 Overview of activity rate data and mercury input factor types needed to estimate releases from geothermal power production
Life-cycle phase
|
Activity rate data needed
|
Mercury input factor
|
Geothermal power production
|
Mwe/hour
|
Grams (g) Hg released per Mwe/hour
|
-
Important input factors include an estimate of the energy production in megawatt (Mwe) per hour and an estimate of the amount of mercury mobilized per megawatt hour (g Hg/Mwe/hr).
5.1.7.4Examples of mercury in releases and wastes/residues -
For off-gas ejectors the US EPA presents a range of atmospheric emissions factors of
0.00075 - 0.02 grams of mercury per megawatt hour (g/Mwe/hr) with an average of 0.00725 g Hg/Mwe/hr. For cooling towers, EPA presents a range of 0.026 - 0.072 g Hg/Mwe/hr for air emissions factors with an average of 0.05 g/Mwe/hr (US EPA, 1997a). However, these factors are based on limited emissions data obtained in 1977 in the USA and process information was not provided and the data have not been validated. Therefore, the emissions factors should be used with caution (US EPA, 1997a).
5.1.7.5Input factors and output distribution factors -
No attempt was made so far to develop default input and output factors for this sub-category. If no specific data are available, release estimates might be based on the information given above.
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